The semiconductor industry currently uses different types of semiconductor-based image sensors that use micro-lenses, such as charge coupled devices (CCDs), CMOS active pixel sensors (APS), photodiode arrays, charge injection devices and hybrid focal plane arrays, among others. These image sensors use micro-lenses to focus electromagnetic radiation onto the photo-conversion device, e.g., a photodiode. Also, these image sensors can use filters to select particular wavelengths of electromagnetic radiation for sensing by the photo-conversion device.
Micro-lenses of an image sensor help increase optical efficiency and reduce cross talk between pixel cells. FIG. 1A shows a portion of a CMOS image sensor pixel cell array 100. The array 100 includes pixel cells 10, each being formed on a substrate 1. Each pixel cell 10 includes a photo-conversion device 12, for example, a photodiode. The illustrated array 100 has micro-lenses 20 that collect and focus light on the photo-conversion devices 12. The array 100 can also include a light shield, e.g., a metal layer 7, to block light intended for one photo-conversion device from reaching other photo-conversion devices of the pixel cells 10.
The array 100 can also include a color filter array 30. The color filter array includes color filters 31a, 31b, 31c, each disposed over a pixel cell 10. Each of the filters 31a, 31b, 31c allows only particular wavelengths of light to pass through to a respective photo-conversion device. Typically, the color filter array 30 is arranged in a repeating Bayer pattern and includes two green color filters 31a, a red color filter 31b, and a blue color filter 31c, arranged as shown in FIG. 1B.
Between the color filter array 30 and the pixel cells 10 is an interlayer dielectric (ILD) region 3. The ILD region 3 typically includes multiple layers of interlayer dielectrics and conductors that form connections between devices of the pixel cells 10 and from the pixel cells 10 to circuitry (not shown) peripheral to the array 100. Between the color filter array 30 and the micro-lenses 20 is a dielectric layer 5.
Typical color filters 31a, 31b, 31c can be fabricated using a number of conventional materials and techniques. For example, color filters 31a, 31b, 31c can be a gelatin or other appropriate material dyed to the respective color. Also, polyimide filters comprising thermally stable dyes combined with polyimides have been incorporated into photolithography processes. Although color filters prepared using photolithography can exhibit good resolution and color quality, photolithography can be complicated and results in a high number of defective filters 31a, 31b, 31c. Specifically, using photolithography to form the color filter array 30 including polyimide filters 31a, 31b, 31c requires a mask, a photoresist, a baking step, an etch step, and a resist removal step for each color. Thus, to form color filter array 30 arranged in a Bayer pattern, this process must be repeated three times.
It would, therefore, be advantageous to have alternative filters for use in an image sensor to provide a greater variety of engineering and design opportunities.